1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing a large, double-sided, curved Fresnel lens that has a Fresnel groove on each surface.
2. Description of the Related Art
FIG. 1A schematically shows a large, double-sided, curved Fresnel lens. It is proposed to use an optical system having two large, double-sided, curved Fresnel lenses 2 and 2′ for imaging in-order to observe light (Cherenkov radiant ray) generated from high-energy particles at the time the high-energy particles fall in the atmosphere from outside the earth. By observing Cherenkov radiant ray, it is possible to measure strength, a wavelength and energy of an elementary particle that falls to the earth.
The large, double-sided, curved Fresnel lenses 2 and 2′ are launched together with a space shuttle into a stationary orbit of the earth to observe elementary particles at the range corresponding to the area of a diameter of about 600 km on the earth's surface. A very wide-sighted refractive optical system that can measure elementary particles includes two large, double-sided, curved Fresnel lenses 2 and 2′ each of which has a diameter of 2500 mm. One Fresnel lens 2 includes two spherical surfaces S1 and S2 that have radii of 4099 mm and that are separated from each other by the distance of 20 mm in the optical axis direction. Fresnel grooves are formed on the surfaces S1 and S2, respectively.
The other Fresnel lens 2′ includes two spherical surfaces S4 and S5 that have radii of 2555 mm and that are separated from each other by the distance of 20 mm in the optical axis direction. Fresnel grooves are formed on the surfaces S4 and S5.
In order to manufacture the large, double-sided, curved Fresnel lenses 2 and 2′ having the diameters of 2500 mm, it is necessary to prepare a ultra-precise machining apparatus having a large rotary table of which diameter is no smaller than 2500 mm. Further, in the future, when a further larger, double-sided, curved Fresnel lens is required, the ultra-precise machining apparatus has to be made larger in size. In addition, a reinforcing tool needs to be attached to the large lens to increase the lens strength. Therefore, it is not easy and realistic to develop such a large ultra-precise machining apparatus.
For this reason, as schematically shown in FIGS. 1A and 1B, it is assumed to divide the large, double-sided, curved Fresnel lenses 2 and 2′ into a plurality of segment lenses 3. That is, it can be considered to manufacture a predetermined number of segment lenses 3 having specific sizes. In this example, three or four types of segment lenses 3 needs to be manufactured.
Specifically, in the example of FIG. 1A, each of the two lenses 2 and 2′ are assembled by arranging a center lens 4 having a diameter of 1500 mm, twenty-four inside tablet slice segment lenses 3 and twenty-four outside tablet slice segment lenses 3 each of which corresponds to a circle part equally divided from one circle by 15 degrees. In this manner, by manufacturing small components and putting them together, it is possible to ease transportation difficulty arising when one large Fresnel lens is manufactured.
One tablet slice segment lens 3 needs to be formed such that the back sides 5a and 6a of the Fresnel lens surfaces 5 and 6 have uniform heights of 1 mm, and the segment lens 3 needs to have an entirely uniform thickness “t” of 20 mm in the direction of the lens optical axis Z. Further, the lens 3 needs to have an entire shape of a surface of revolution around the optical axis Z, and the optical axis Z of the Fresnel surface at the front side needs to conform to the optical axis Z of the Fresnel surface at the back side. Additionally, the height of the back sides 5a and 6a of the lens 3 is indicated by “h” of FIG. 1C.
Furthermore, the Fresnel lens is required to have high transparency, highly precise Fresnel lens groove, and high lens quality because of its use application.
The Fresnel lens has the structure in which refractive surfaces are distributed to narrow band-shaped regions with the refractive surfaces being flat. Since the outline of the Fresnel lens is thin plate-shaped, it is advantageous to make good use of a small volume and light weight of the Fresnel lens. A mold having Fresnel lens transfer groove engraved on the flat surface thereof is used together with a flat plate for sandwiching a Fresnel lens material between the mold and the flat plate to fabricate the Fresnel lens by heat pressing. Accordingly, the Fresnel lens is formed on the one side surface of the material.
Recently, in order to take more advantage of characteristics of the thin plate-shaped Fresnel lens and more freely design the Fresnel lens, it has been desired to form the Fresnel lenses on both sides of a curved plate. Two plate materials each of which has the Fresnel lens surface on one side thereof are combined to each other on the other flat surfaces thereof to assemble a thin, flat, double-sided, plate-shaped Fresnel lens.
However, there has not been a mold by which the Fresnel grooves are formed on both sides of a curved plate. Accordingly, it has been difficult to produce a curved lens having Fresnel grooves on both sides thereof.